1. Field of the Invention
The present invention relates to an engine valve operation timing control apparatus for varying operation timing of intake and exhaust valves of an engine.
2. Description of Related Art
A valve operation timing control apparatus is used in a timing control system to advance or retard operation timing of intake and exhaust valves of an engine as disclosed, for example, in Japanese Patent Application Laid-Open No. Hei. 1-134010. The apparatus includes a phase control member, engaged between a crankshaft and a camshaft of the engine, which varies the operation timing of intake and exhaust valves driven by rotors on the camshaft by varying a rotational phase between both shafts by sliding the phase control member using two hydraulic systems.
However, the valve operation timing control apparatus described above has problems in that, because it requires two hydraulic systems to slide the phase control member, its structure is complicated and in that, because the phase angle between the crankshaft and the camshaft is varied simply by controlling the opening and closing of an electromagnetic valve, it is difficult to control a small angle of timing advance.
In order to solve the aforementioned problems, Japanese Patent Laid-Open Publication No. Hei. 6-299813 discloses a valve operation timing control apparatus whose structure is simplified by unifying the hydraulic systems of the valve operation timing control apparatus and which allows control of a small angle of timing advance by continuously controlling an opening of the electromagnetic valve of the hydraulic system and by adopting a feedback learning control feature for the control of the opening of the electromagnetic valve. This apparatus is arranged so that a control value which is output from a controller to the electromagnetic valve of the hydraulic system is stored in learning means when a target value of a rotational phase angle between the crankshaft and the camshaft is almost constant for a predetermined time, and the control value stored in the learning means is used as a learned value in generating a control value.
This apparatus assumes as its principle of learning that the rotational phase difference converges almost to a constant value when a state in which the target value of the rotational phase difference between both shafts is almost constant continues for a predetermined time and adopts as a condition of the learning that the target value is almost constant for the predetermined time. However, it is possible that, in the hydraulic unit, a leak increases due to a pressure drop and a drop in viscosity of control oil when the engine speed is low and oil temperature is high. In this case, a change speed of the rotational phase difference drops and an actual rotational phase difference (real phase difference angle) is still changing even if the target value is almost constant and the predetermined time has passed. If the predetermined time is prolonged in order to deal with this problem, the learning operation becomes difficult to perform, thereby reducing the learning frequency and preventing the system from dealing adequately with changes in operating conditions.